fbdev: fix potential buffer overflow in do_register_framebuffer()

The current implementation may lead to buffer overflow when:
1.  Unregistration creates NULL gaps in registered_fb[]
2.  All array slots become occupied despite num_registered_fb < FB_MAX
3.  The registration loop exceeds array bounds

Add boundary check to prevent registered_fb[FB_MAX] access.

Signed-off-by: Yongzhen Zhang <zhangyongzhen@kylinos.cn>
Signed-off-by: Helge Deller <deller@gmx.de>

Author: Yongzhen Zhang

drivers/video/fbdev/core/fbmem.c

@@ -449,6 +449,9 @@ static int do_register_framebuffer(struct fb_info *fb_info)
 		if (!registered_fb[i])
 			break;
 
+	if (i >= FB_MAX)
+		return -ENXIO;
+
 	if (!fb_info->modelist.prev || !fb_info->modelist.next)
 		INIT_LIST_HEAD(&fb_info->modelist);